1. Field of the Invention
The present invention relates to encoding methods and apparatus for digital magnetic recording systems and particularly to systems for saturation recording of closely packed data. The invention is described herein with reference to a system utilizing quaternary data encoding with freedom in selection of three independent variable characteristics.
2. Description of the Prior Art
It has been recognized in the art of magnetic recording that information density stored on the medium may be enhanced by packing magnetic flux transitions as closely as possible. However, as data is compressed in recording, interaction between successive bits at high density results in intersymbol interference and nonlinear distortion. For a given transducer/medium interface, which dictates the maximum flux density attainable, improved coding methodology derived from information theory and modern communications techniques has been applied successfully to provide effective bit density increase over a conventional encoding methodology. The "code efficiency" (CE) defined as the ratio between effective bit density and actual flux density, is thereby increased.
In digital magnetic recording, most conventional codes such as Double Frequency (DF), Modified Frequency Modulation (MFM or Miller), etc. convey the original information exclusively via the recorded flux transition positions; during data recovery the encoded transition position is found from the peak (or zero-crossing after differentiation) of the read signal. See, for example, N. D. Mackintosh, The Choice of a Recording Code, IERE Conf. Proc. No. 43, p. 77 (1979). In these codes, the flux transition alternates as it traverses the recorded bit string. Only the precise location, not the transition polarity, can be used to convey information in the present day saturated digital magnetic recording systems. The Three Position Modulation (3PM) code invented by G. Jacoby and described in U.S. Pat. No. 4,323,931, issued Apr. 6, 1982, which gives a 50 percent code efficiency improvement over MFM by re-distribution or modulation of flux transition locations (CE=1.5), is essentially a member of the one-variable runlength limited code group--in terms of degrees of freedom available in recorded signal variables for information encoding. One variable code groups, such as the above, operate on the principle of encoding the input data on position variations. The present invention, however, is a member of the group codes, whereby groups of bits are coded with unique patterns. Heretofore the group codes conveyed the original information either on position and waveshape of recorded flux transitions. (Ternary Data Encoding System as disclosed in U.S. patent application Ser. No. 260,248, filed May 5, 1981, in the name of G. V. Jacoby and M. Cohn), or on the polarity and presence or absence of a break (as disclosed by C. S. Chi, U.S. patent application Ser. No. 339,352, filed Jan. 15, 1982, Controlled Return to A.C. Digital Magnetic Recording and Reproducing System). Said Ser. No. 260,248 and Ser. No. 339,352, both assigned to the assignee of the present invention, give a potential 50% density improvement over MFM, and can be called "two-variable" codes since two independent degrees of freedom are available for original information encoding. While a two-variable code provides enhanced efficiency over the one-variable codes, more efficient three-variable codes have heretofore not been available.
It is a compelling requirement that any encoding methodology used provides self-clocking; that is, synchronization pulses which can be derived directly from the readback pulses, for high data reliability. Since the data is recovered relative to a time oriented window, as the data packing density is increased, the tolerance in timing error decreases. Thus, coding schemes may impose run length constraints, so that where the clock signal is derived from the recorded transitions long runs of binary 0's, which do not result in transitions, are precluded, thus avoiding drift of the timing clock. The NRZ code, for example, cannot be used directly in modern magnetic recording systems due to the lack of self-clocking and its demand on extended low-frequency response for "worst case" data patterns of uninterrupted strings of 1's or 0's.